Hi Maya,
There are no known reasons for this to to occur. Would you please provide more information on your steps? For example, did you first upgrade the 10.0 fabric to 10.1 fabric from the General property page for the fabric properties, or are you working on the same 10.0 fabric?
When you are looking for the plans, are you opening the Plans table directly, (for example by dragging and dropping it from the catalog window, and then clicking open from the table of contents), or are you using the Plan Directory, via the Parcel Editor toolbar: Parcel Editor-> Plan Directory?
Did you notice the missing plans immediately, or did you edit and save edits on the fabric before you noticed the missing plans?
Did you do any new data migration from source data into your fabric after you started using 10.1?
Thanks,
Tim (Parcel editing team)